Runaway Trains and Gremlins- This Story Is Out of Control

My current work in progress started out as something cute.  It was a fuzzy, warm, inspiring sort of thing.  It has turned into an entirely different creature.  In writing this, I am reminded of the Gremlins movies.  The little things are all adorable and harmless- but  never expose it to bright light, never get it wet, and never feed it after midnight..

Well, my story spent the day in the sun, under the sprinkler, and had one hell of a midnight snacking binge.  It has grown teeth, parts of it are slimy and evil, and I swear it just growled at me.  My characters are confused and depressed- and often there is nothing I can do to help them.  Just when I think the story is about to turn back into that fluffy creature- it rains, someone leaves the Twinkies out, and the story turns into a monster.

What I had set out to write in a light, warm spirit has actually become rather dark and serious.  I don't know if I can redeem the tone- or if I even want to at this point. Even though I plot and plan, I often feel that when it comes right down to it I'm not really the one in control.

I'm interested to know if others have this experience.  Does your story ever run away with you? If it does, do you whip it back into shape, or do you let it take you someplace else?